End-of-ribbon sensor circuitry

ABSTRACT

End-of-ribbon sensor circuitry for use with an end-of-ribbon sensor of an electronic typing system comprises an input voltage divider for receiving the output signal of the end-of-ribbon sensor. The two signals from the voltage divider are applied to a differentiating circuit providing different time constants to the two signals. The differentiating circuit compensates for varying conditions of ambient light within the ribbon cartridge and provides a delay which eliminates false signals due to rapid fluctuations of the ribbon. After differentiation, the two signals are compared utilizing delayed switching time to determine the presence of an end-of-ribbon signal. The output, if any, of the comparator is applied to a sampling circuit where the signal is sampled for a predetermined period of time after the print hammer is fired. If an end-of-ribbon signal is present, said signal is clocked into a latching circuit from which it is sent to the printer control system which stops the printer when the present line of print is completed.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to and includes similar disclosure tocopending U.S. Application Ser. No. 787,794 filed concurrently herewithin the name of Jerry W. Hedstrom for End-of-Ribbon Sensor Device andassigned to the assignee of the present invention.

BACKGROUND OF THE INVENTION

This invention relates, in general, to ribbon sensing apparatus and moreparticularly to circuitry used in conjunction with an end-of-ribbonsensor device for providing an indication of when the ink ribbonremaining on the supply spool has been depleted to a predeterminedamount, such as the end portion.

In various commercially available typewriters and other data printingmachines employing an ink ribbon in the printing process, it is normallydesirable to provide means to indicate when the ink ribbon isessentially depleted from the supply spool. This requirement is moreimportant when a matrix-type plastic ribbon is used rather than a clothribbon since the plastic ribbon is used only one time during theprinting function. With the advent of the electronically controlledautomatic text-editing typewriter employing a ribbon cartridge, it iseven more important to provide such indicating means. Without some typeof indication or control regarding the amount of ink ribbon remaining onthe supply spool, a portion of a line or page could be automaticallytyped without the benefit of an ink ribbon.

It has been known to employ various types of physical pins or clipsattached to the ribbon near the end of the ribbon supply to eitherphysically move a lever or to provide electrical contact to sense theend portion of the ribbon remaining on the ribbon supply. It is alsoknown to employ rod or lever means, which project through an opening inthe ribbon supply hub upon the depletion of the ribbon on the ribbonsupply spool, to sense the end portion of the ribbon. In addition, it isknown to employ a lever or follower which contacts the outer peripheryof the ink ribbon on the supply spool and through its pivotal motionduring the depletion of the ink ribbon, signals the occurrence of theend portion of the ribbon on the ribbon supply spool. The previouslydiscussed types of sensing mechanisms have been implemented in machineshaving an ink ribbon mechanism consisting of a ribbon supply spool and aribbon take up spool mounted on opposite sides of the machine ratherthan being contained within a ribbon cartridge.

With machines employing the moving carriage with a ribbon cartridgemounted thereon, a smaller and less mechanically complex type of sensingmechanism is desired which may be mounted on the carriage for movementtherewith. It would also be desirable to have the end-of-ribbon sensorbe free of physical contact with the ribbon and thereby eliminate theproblem of having the end-of-ribbon sensor become a factor in thedetermination of ribbon drive force or ribbon tension force required forthe ribbon system. Presently, there is marketed a serial printer whichemploys a movable carriage with the ribbon cartridge and the daisy wheelprint element mounted thereon. When an end-of-ribbon sensor is embodiedin such an existing printer, which is presently embodied in acommercially available automatic text editing typewriter, it isdesirable to provide such a sensor without requiring a significantredesign of the carriage and associated elements. When positioning anend-of-ribbon sensor on or attached to the moving carriage, whichincludes the ribbon cartridge, the sensor should not restrict the motionof the carriage or associated elements or cause appreciable dynamicproblems due to its mass or friction. If the sensor is of the opticaltype, the sensor should function properly with different types ofribbons and different colors of ribbon and function properly undervarious light conditions normally found in the office environment. It isalso desirable that the end-of-ribbon sensor provide an indication thatthe end of the ribbon is near but provide the indication at a pointwhich allows the present line of typing to be completed prior tostopping the printer.

Circuitry associated with the end-of-ribbon sensor should provide areliable and positive signal indicating the presence of the end portionof the ribbon.

Accordingly, it is a primary object of the present invention to provideimproved circuitry to be used with an end-of-ribbon sensor with thecircuitry providing a positive indication of the presence of the end ofthe ribbon.

Another object of this invention is to provide improved circuitry whichincorporates components to introduce delay and comparison features toimprove reliability of the indication of the end of the ribbon.

A further object of the present invention is to provide capability ofsampling of the indication of the end-of-ribbon at specific times toimprove reliability of the indication.

Other objects and advantages will be evident from the specification andclaims when read in conjunction with the accompanying drawingillustrative of the invention.

SUMMARY OF THE INVENTION

In accordance with the principles illustrative of this invention, theforegoing objects and others of the present invention are accomplishedby the provision of an end-of-ribbon sensor circuitry comprising aninput voltage divider followed by a differentiating circuit. The stepvoltage from the end-of-ribbon sensor is applied to the voltage divider.The two signals provided from the voltage divider are applied to thedifferentiating circuit which provides different time constants to thetwo signals. After differentiation, one of the two signals is applied tothe + terminal of an operational amplifier while the second signal isapplied to the - terminal of the operational amplifier. The operationalamplifier is turned on when the value of the voltage at the + terminalexceeds the value of the voltage at the - terminal. This operationprovides a comparator function of the two applied voltages from thedifferentiating circuit with the different time constants providing adelayed switching time thereby increasing the reliability of theend-of-ribbon signal. The output of the operational amplifier is appliedto a sampling circuit where the signal is sampled for a predeterminedperiod of time after the print hammer is fired. If an end-of-ribbonsignal is present during the sample time, this signal is applied to alatch circuit which sets the end-of-ribbon signal and sends the signalto the printer control system which stops the printer when the presentline of print is completed.

BRIEF DESCRIPTION OF THE DRAWING

Other advantages and features of the present invention may become moreapparent from reading the following detailed description in connectionwith the drawing forming a part thereof, in which:

FIG. 1 is a top plan view of a printer embodying the present invention;

FIG. 2 is a side plan view of the carriage of the printer of FIG. 1;

FIG. 3 is a simplified perspective view of the end-of-ribbon sensordevice;

FIG. 4 is a simplified side sectional view of the end-of-ribbon sensordevice taken generally along line 4--4 of FIG. 3 and also showing thepositional relationship of the sensor device with respect to the ribbon;

FIG. 5 is a simplified perspective view of the ribbon cartridge mountingplate embodying the end-of-ribbon sensor device;

FIG. 6 is a simplified perspective view of the reflective tapepositioned on the ribbon;

FIG. 7 is a simplified top view of the ribbon cartridge positioned onthe mounting plate and with a portion of the top cover removed todisclose the end-of-ribbon sensor device positioned within the ribboncartridge;

FIG. 8 is a simplified electrical schematic of the end-of-ribbon sensordevice;

FIG. 9 is a simplified schematic of the circuit used in conjunction withthe end-of-ribbon sensor device; and

FIG. 10 shows two waveforms of voltages appearing in the schematic ofFIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing and more particularly to FIG. 1, an overallview of the printer 50 embodying the present invention is illustrated.Mounted on a base frame 57 is a platen 47 with knobs 45 and 46 forrolling the platen 47 and paper record medium (not shown) wrappedthereon. A carriage 10 is mounted for linear movement on the carriagemounting rails 11 and 12 bridged between opposed side walls of the baseframe 57 of the printer 50. Carriage 10 includes a rotary print wheel 13on which are a number of type element or character slugs, a hammer-typeimpact printing mechanism 48 for striking a selected type element, aribbon cartridge 16 having an inked ribbon 24 interposed between thepaper on the platen 47 and the type element on the print wheel 13located at the print position or station and a ribbon advance motor 17as best shown in FIG. 2.

Furthermore, there are a stepping motor 31 and a servo motor 32 mountedon the base frame 57. The stepping motor 31 is coupled to the platen 47by a gear train 33 so that the platen 47 is indexed when the motor 31 isactivated to incrementally advance the paper through the printer 50. Theservo motor 32, on the other hand, has one end of its drive shaft 34coupled to the carriage 10 by a cable 35, which is trained around aseries of pulleys 36-40, and the other end of its drive shaft 34 coupledto a shaft encoder 41. Thus, the carriage 10 is moved to translate theprinting mechanism 48 lengthwise of the platen 47 when the servo motor32 is actuated, while the encoder 41 supplies a signal which isrepresentative of the actual position of the carriage 10 at any giventime.

With reference to FIGS. 1 and 2 and the printer carriage 10 mounted forlinear movement on the mounting rails 11 and 12, as typing of theprinted characters occurs, the carriage 10 stops each time a characteris to be printed. Also, while the carriage 10 is moving from onelocation to the next location along rails 11 and 12 by movement of thecable 35, the print wheel 13 is rotated such that the next character tobe printed will be in position at the print position or station when thecarriage 10 stops and the printing mechanism 48 is fired. As seen inFIG. 2, the upper portion of carriage 10 is pivotable clockwise aboutshaft 14 with respect to the lower portion of carriage 10. This pivotingmotion is necessary in order to bring the print wheel 13 up into aposition such that the print wheel 13 may be exchanged for a differentprint wheel 13.

The ribbon cartridge mounting plate 15 provides the mounting structurefor the ribbon cartridge 16, the ribbon advance motor 17 and the ribbonadvance gearing 18. The ribbon cartridge mounting plate 15 is pivotableclockwise, in FIG. 2, about shaft 19. This pivoting motion is necessaryto raise the ribbon 24 from the down position, which is the positionthat allows the printed material to be viewed by the operator, to the upposition at the print station when printing is to occur. The force topivot the ribbon cartridge mounting plate 15 is a magnetic forcesupplied by electromagnet coils 20. When coils 20 are energized, thatportion of the ribbon cartridge mounting plate 15 above coils 20 isdrawn down toward the coils 20 thereby raising that portion of theribbon cartridge 16 which is nearest the print wheel 13 to the printposition.

Still referring to FIGS. 1 and 2, carriage 10 also includes a motor 23having a shaft 28. Mounted on one end of the shaft 28 is the rotaryprint wheel 13. Wheel 13 includes a central cap portion 29 constructedof rubber with a stiffening ring which allows the wheel 13 to be easilyremoved from the shaft 28 and, for example, replaced with another wheel13 with, for example, a different font of characters. The other end ofshaft 28 has mounted thereon a transducer 60 which provides positionsignals related to the rotary positions of the shaft 28 and, therefore,the print wheel 13. Transducer 60 includes a fixed disc 62 adjacent adisc 64 mounted for rotation with the shaft 28. Electrical interactionbetween these two discs 62 and 64 produced the position signals whichare used in a servo system for controlling the print wheel 13.Consequently, when motor 23 is activated with the necessary signal, theprint wheel 13 is rotated as necessary to bring any selected one of itscharacter slugs into alignment with the hammer printing mechanism 48 forprinting out the selected character. Additionally, the transducer 60supplies a continuously updated signal which is representative of theactual position of the print wheel 13.

Referring now to FIGS. 3 and 4, there is shown a simplified view of theend-of-ribbon sensor 70 for providing indication of the end portion ofthe ink ribbon 24 of the present invention. End-of-ribbon sensor 70comprises a housing 72, which is cylindrical in the present embodiment,with a closed end portion 74. Housing 72 protrudes essentially at rightangles from mounting plate 73, and the plane formed thereby, which iscoupled to a substantially square mounting block 75. Cutouts 76 and 77in block 75 cooperate with ribbon cartridge mounting plate 15 and anopening or cutout 22 formed therein to position sensor 70 on the ribboncartridge mounting plate 15. Aperture 79, in mounting plate 73,cooperates in mounting the sensor 70 on the ribbon cartridge mountingplate 15. A light source 81 and a light detector 83 are positionedwithin housing 72 and cooperate through apertures 84 and 86 respectfullyin the end-of-ribbon sensing function. The light source 81 may be anyone of the semi-conductor type light emitters; however, a suitableexample could be a type known as the solution-grown epitxial galliumassenide light emitting diode. Commercially available diodes which havebeen found to give the desired results are the light emitting diodesSE-1450 Series and SE-2450 Series, available from SpectronicsIncorporated, which emit narrow-spectrum radiant energy in the nearinfrared region of the spectrum. Light detector 83 may be any one of thesemi-conductor photo-voltaic cells or photo-transistors having aspectral response characteristic similar to that of the light source 81.A particular type of photo-transistor formed to give good results inthis particular embodiment are photodarlingtons (silicon) SD-1410 Seriesand SD-2410 Series, available from Spectronics Incorporated. Washer 87may be positioned down and over housing 72 to rest on and against plate73. Washer 87 is preferably of a felt-type material which provides anadditional light-sealing function between sensor 70 and ribbon cartridge16 when the sensor 70 is in operative position with housing 72 beingwithin the interior volume of ribbon cartridge 16. Electricalconnections to the sensor 70 are made via electrical leads 88-90.

Referring now to FIG. 4, there is shown the positional relationship ofthe sensor 70 and the ribbon 24. The diameter of apertures 84 and 86 ispreferably about 0.06 inches. The distance between horizontalcenterlines of apertures 84 and 86 is preferably about 0.09 to 0.10inches. The position of the apertures 84 and 86 along the length ofhousing 72 and the length or height 72A of housing 72 projecting abovemounting plate 73 is dependent upon the height 24A of the ink ribbon 24and the position of the ribbon 24 with respect to mounting plate 73.Preferably the length or height 72A of housing 72 and the position ofapertures 84 and 86 will be such that the beam of light from lightsource 81 will strike approximately in the center of height 24A ofribbon 24 and be reflected back to light detector 83. In the particularembodiment described, height 72A is about 0.46 inches and the verticaldistance or height 82, which is the distance from mounting plate 73 tothe center point between apertures 84 and 86, is about 0.27 inches. Thediameter 72B of housing 72 is preferably about 0.150 inches.

The beam of light from light source 81 is most intense in the center andconsiderably reduced in intensity off the center axis. Likewise, thesensitivity of the light detector 83 is greater to light received alongthe center of its axis of sight and considerably reduced in sensitivityto light received from off the center axis. In order to assure themaximum sensitivity of the overall operation of the sensor 70, the lightsource 81 and the light detector 83 are tilted or canted with respect toa plane passing through the center point between apertures 84 and 86,with said plane being parallel to the plane formed by mounting plate 73.The light source 81 is tilted or canted in an upward direction by anangle α from a plane passing through the centerpoint between apertures84 and 86 and parallel to the plane formed by mounting plate 73 whilethe light detector 83 is tilted or canted in a downward direction by anangle B from said plane. Angles α and B may be varied over the rangefrom 0° to 16° with the optimum preferred value being 8° for both α andB. With the distance 85 between the ribbon 24 and the verticalcenterline of housing 72 being about 0.380 inches, the 8° of tilt to thecenter axis of both the light source 81 and the light detector 83results in the center lines of both the light source 81 and lightdetector 83 intersecting at the ink ribbon 24. This intersecting resultsin the maximum amount of light being reflected along the center axis oflight detector 83.

With reference to FIG. 5, there is shown the ribbon cartridge mountingplate 15 with the end-of-ribbon sensor 70 mounted thereon. Prior to thepresent invention, the ribbon cartridge 16 was positioned on the ribboncartridge mounting plate 15 by two guide pins positioned to interfacewith two compatible apertures in the lower cover section of the ribboncartridge 16. Guide pin 21 in FIG. 5 is one of such prior positioningand alignment pins which remains on the ribbon cartridge mounting plate15. The other guide pin was removed and replaced by the end-of-ribbonsensor 70 which was designed to have substantially the same diameter andheight as the guide pin which it replaced; the end-of-ribbon sensor 70is part of the mounting means for the ribbon cartridge 16. By designingthe end-of-ribbon sensor 70 in this manner, the sensor 70 could beincorporated into the existing printer 50 without the need for a majorredesign of the present printer 50 or the ribbon cartridges 16. Cutout22 was formed in the ribbon cartridge mounting plate 15 and co-acts withcutouts 76 and 77 of mounting block 75 of the end-of-ribbon sensor 70 toassist in the mounting of sensor 70 to the ribbon cartridge mountingplate 15. Screw or bolt 25 in cooperation with aperture 79 and acorresponding aperture in the ribbon cartridge mounting plate 15complete the mounting means for mounting sensor 70 on the mounting plate15.

Referring now to FIG. 6, a portion of ink ribbon 24 is shown with asuitable marker 26 attached thereto for use in conjunction withend-of-ribbon sensor 70. The marker 26 has a reflective surface on thatside of the marker 26 positioned away from the ribbon 24. The marker 26is attached by suitable adhesive to the side of the ribbon 24 positionedaway from the platen 47, FIG. 1, and at a predetermined distance fromthe physical end of the ribbon 24 such that when the marker 26 isdetected, enough ribbon 24 will be available to complete the printing ofany line which has been started. The marker 26 is substantially the sameheight as ribbon 24 and is approximately one-half inch in length. Thematerial of the marker 26 in the disclosed embodiment is aluminizedpolyester. The scan side of ribbon 24 shall have a 300 NM to 1000 NMintegrated reflection of less than 5% of the integrated 300 NM to 1000NM of the aluminum polyester marker 26. With regard to reflectiveproperties, ribbon 24 provides a low peaked response to light source 81while marker 26 provides a high broad response.

With reference to FIG. 7, the ribbon cartridge 16 is shown with aportion of the top cover removed to disclose the end-of-ribbon sensor 70positioned within the interior volume of the ribbon cartridge 16. Inaddition, the positional relationship between the end-of-ribbon sensor70 and the ribbon 24 is shown. Regarding ribbon path, the ribbon 24passes from the ribbon supply means (not shown) past the end-of-ribbonsensor 70 and vanes 42 and 43 and out an aperture in horn 27, across thespace between horns 27 and 30, into an aperture in horn 30 and then onto the ribbon take up means (not shown). Vanes 42 and 43 are positionedon either side of the end-of-ribbon sensor 70, with reference to thedirection of travel of ribbon 24 past sensor 70, and are oriented at 90degrees to the ribbon path. Vanes 42 and 43 extend from wall 51 to apredetermined distance from wall 52 to limit any flutter which occurs inribbon 24 during the travel of ribbon 24. Vanes 42 and 43, inconjunction with wall 52 maintain the distance from the longitudinalcenterline of end-of-ribbon sensor 70 to the ribbon 24 to approximately0.38 inches, as shown in FIG. 4. The reduction of flutter of ribbon 24reduces the range of signal levels from the end-of-ribbon sensor 70 whenthe ribbon 24 is being sampled for the presence of marker 26 and therebyreduces any false indications received. In addition, vanes 42 and 43contribute to the reduction of any external light entering the ribboncartridge 16 in the location of the end-of-ribbon sensor 70 and theribbon 24. This reduction of background light tends to increase theaccuracy and dependability of the overal function of end-of-ribbonsensing. The end-of-ribbon sensor 70 protrudes into the interior volumeof ribbon cartridge 16 to provide the positional relationship with theribbon 24 as shown in FIG. 4.

Referring now to FIG. 8, there is shown the simplified electricalschematic of the end-of-ribbon sensor 70. Light detector 83 comprisestransistors 92 and 93. Transistor 92 has a light base electrode and anemitter electrode connected to the base electrode of transistor 93.Collector electrodes of transistors 92 and 93 are connected together andto terminal 96 to which a power supply voltage (not shown) such as a +5volts, is supplied. The negative terminal of the power supply (notshown) is connected to terminal 97. Emitter electrode of transistor 93is connected to terminal 95. The anode electrode of light source 81 isconnected to terminal 96 while the cathode electrode is connected toterminal 97 through resistor 44, which biases light source 81. A loadcomprising the end-of-ribbon sensor circuitry 100, which will bediscussed later, is connected between terminals 95 and 97. In operation,energy radiated from light source 81 is reflected from the ribbon 24 orthe marker 26, if the ribbon 24 in the ribbon cartridge 16 has been usedto that position, and activates transistor 92 which in turn activatestransistor 93. If the ribbon 24 reflects the energy, the end-of-ribbonsensor 70 provides approximately 0.2 milliamperes to the sensorcircuitry 100 while sensor 70 supplies approximately 1.3 milliamperes tosensor circuitry 100 if the marker 26 is in position to reflect theenergy to light detector 83.

With reference to FIG. 9, there is shown the simplified schematic of theend-of-ribbon sensor circuitry 100 used in conjunction with theend-of-ribbon sensor 70. Resistors 101 and 102 are connected in seriesacross terminals 95 and 97. Resistor 104 is connected from terminal 95to the negative input terminal of operational amplifier (op amp) 107.Resistor 103 is connected from the junction point between resistors 101and 102 to the positive input terminal of op amp 107. Capacitor 105 isconnected from terminal 97 to the positive input terminal of op amp 107while capacitor 106 is connected from terminal 97 to the negative inputterminal of op amp 107. The output of op amp 107 is connected toterminal 111 through resistor 108. Diode 109 is connected from terminal111 to V₄ to which is applied, for example, +5 volts. Diode 110 isconnected from terminal 111 to ground and/or terminal 97 which isgrounded. Plus 15 volts are applied to the V₂ terminal of op amp 107while minus 15 volts are applied to the V₃ terminal of op amp 107.Resistors 101 and 102 form a voltage divider for the output of theend-of-ribbon sensor 70. In the disclosed embodiment, resistor 101 has avalue of 1.5 K ohms and resistor 102 has a value of 1 K ohms. Thiscombination results in the voltage at the junction between resistors 101and 102 being two-fifths the value of the voltage at terminal 95 acrossboth the resistors 101 and 102. These two voltages are filtered byrespective filters, with the voltage at terminal 95 being filtered byresistor 104 and capacitor 106 prior to application to the op amp 107.In the disclosed embodiment, resistor 104 is 390 K ohms and capacitor106 is 22 mfd. giving a time constant of approximately 8 seconds. Thevoltage at the junction between resistors 101 and 102 is filtered byresistor 102 and capacitor 105 prior to application to op amp 107. Inthe disclosed embodiment, resistor 103 is 5.1K ohms and capacitor 105 is10 mfd. giving a time constant of approximately 50 milliseconds. Thetime constant of resistor 104 and capacitor 106 is so long with respectto variations in the voltage at terminal 95 that the voltage applied tothe negative input terminal of op amp 107 is effectively the average ofthe previous 8 seconds. The voltage applied to the positive inputterminal of op amp 107 is two-fifths of the instantaneous value of thesignal at terminal 95 with a 50 millisecond time constant low passfilter to remove unwanted noise. The applicable equations are:

    V.sub.(neg) = V.sub.95 (1 - e(t/1.sub.1)                   (1)

    V.sub.(pos) = 2/5 V.sub.95 (1 - e(t/1.sub.2)               (2)

Where T₁ = (capacitor 106)(Resistor 104) T₂ = (capacitor 105)(Resistor103)

V.sub.(neg) is the voltage at the - terminal of op amp 107

V.sub.(pos) is the voltage at the + terminal of op amp 107

V₉₅ is the voltage at terminal 95

The voltage applied to the + terminal of op amp 107 is the referencevoltage. In this embodiment, op amp 107 is used as a comparator; op amp107 is either completely on or completely off. Op amp 107 is comparingtwo-fifths of the average voltage with the total voltage at terminal 95.The output of op amp 107 is high whenever the voltage at the + terminalis greater than the voltage at the - terminal.

When power is fist applied, the voltages at the - and + terminal of opamp 107 stabilize in 8 seconds and 50 milliseconds, respectively. Thereason for the two time constants is to provide a delayed switching timeso the op amp 107 will delay switching. The delayed switching isprovided so the system will not respond to ribbon flutter or variationin ribbon reflection due to different types of ribbon 24 to be used withthe typing system.

The differentiating circuit comprising resistor 102 together withcapacitor 105 and resistor 104 together with capacitor 106 compensatesfor varying background light and different ribbon materials and ribboncolors to which the end-of-ribbon sensor 70 is exposed. Thedifferentiating circuit biases itself in accordance with the backgroundlight and different ribbon materials and ribbon colors. The 8 secondaverage signal applied to the -- terminal of op amp 107 will change withthe background light and different ribbon materials and color to whichthe end-of-ribbon sensor 70 is exposed and includes the average ofreflected signals from the ribbon 24 as well as the ambient lightintroduced into the end-of-ribbon sensor 70. Ribbon cartridges do leaksome light in by way of the seams of the cartridges but since thiscircuit compensates for varying conditions of ambient light within thecartridge 16, the system works in the range of conditions from havingthe typing system in direct sunlight to having the system in totaldarkness. The switching delay eliminates false end-of-ribbon indicationsdue to rapid fluctuations of the ribbon 24 and/or signal from theend-of-ribbon sensor 70.

Referring to FIG. 10, there is shown the two waveforms associated withthe two time constants discussed above. When the marker 26 is sensed bythe end-of-ribbon sensor 70, the output current changes fromapproximately 0.2 milliamperes to approximately 1.3 milliamperes and thevoltages applied to op amp 107 change as depicted in FIG. 10. Op amp 107switches when the voltage at the + terminal exceeds the voltage at the -terminal, with the switching delay being the time taken for the voltageon the + terminal to rise above the voltage on the - terminal. For thisparticular embodiment, the delay is approximately 13.6 milliseconds.This time delay is sufficient to filter out any fluctuations associatedwith the signal reflected from the ribbon 24.

Referring again to FIG. 9, diodes 109 and 110 limit the excursion of theoutput of op amp 107 and makes the output compatible with and allowsinterface with transistor-transistor-logic (TTL). For the disclosedembodiment, the output of op amp 107 is 5.8 volts when the voltage atthe + terminal is greater than the - terminal and -0.7 volts when the -terminal is greater.

A sample circuit comprising a D flip-flop 91 and an AND gate 94 providea sampled end-of-ribbon signal every two microseconds out of every 30milliseconds during the printing operation. The sample circuit bringsthe print hammer signal and the clock signal together through Dflip-flop 91. The clock pulse provides a 2 microsecond pulse everytimethe print hammer mechanism is fired. The application of the clock pulseand hammer pulse to D flip-flop 91 provide an output on Q which is andedwith hammer pulse and the end-of-ribbon pulse in AND gate 94. The outputof AND gate 94 is high only when the hammer, Q and end-of-ribbon signalsare high and is a 2 microsecond sampled end-of-ribbon signal. Thisprovides additional filtering and reduces the possibility of anerroneous end-of-ribbon shutdown of the typing system.

The sampled end-of-ribbon signal is provided as the input to a latchcircuit comprising J-K flip flop 114. The clock pulse applied to Cterminal of flip flop 114 sets a level at Q terminal which is clocked insequence with the sampled end-of-ribbon signal applied to the J terminalwhich sets the output signal at Q to a high value. NAND gate 115 invertsthe high signal and sends the resulting low level signal to the controlsystem via output terminal 98 for the printing system. The controlsystem recognizes the signal and shuts down the printer 50 at thecompletion of the line of printing.

In summary, the radiation or light reflected from marker 26 caused astep increase in signal from the end-of-ribbon sensor 70. This signalwas filtered and delayed by a predetermined time to assure it is a validsignal. The signal is then fed through a digital filter 91 and 94 tofurther assure the validity and reliability thereof. The resultingsignal is then fed to a clocked latch circuit 114 and on to the controlsystem which shuts down the printer 50 at the end of the line ofprinting.

The operator activates the shutdown printer 50 by opening the coverwhich by an appropriate switch activates the application of a deselectsignal to the K terminal of J-K flip-flop 114 which sets Q to a lowlevel thereby resetting the circuits. Upon closing the cover, afterreplacing the ribbon cartridge 16 with a new one, the system is readiedfor further printing. The signals and operation of the control system isdescribed in copending and commonly assigned United States patentapplications of H. Wallace Swanstrom et al. which were filed Jan. 2,1974, under Ser. No. 429,479 and Oct. 15, 1975, under Ser. No. 622,780,both of which are hereby incorporated by reference.

Although the present invention has been described with reference to apresently preferred embodiment, it will be appreciated by those skilledin the art that various modifications, alternatives, variations, etc.,may be made without departing from the spirit and scope of the inventionas defined in the appended claims.

What is claimed is:
 1. In a serial printer having an end-of-ribbonsensor, an improved end-of-ribbon sensor circuit comprising:a voltagedivider receiving an output signal from the end-of-ribbon sensor; adifferentiating means receiving two output signals from said voltagedivider, said differentiating means including means for varying the tworeceived signals according to different time constants; and a comparatormeans receiving two differentiated output signals from saiddifferentiating means, one of said differentiated output signalsproviding a reference source for said comparator means, said comparatormeans generating an output signal if the other differentiated outputsignal which is compared against the reference source is lower inmagnitude than said reference source, the presence of said output signalfrom said comparator means providing an indication of the end portion ofa ribbon.
 2. Circuitry as set forth in claim 1 wherein said differenttime constants are approximately 8 seconds and approximately 50milliseconds.
 3. Circuitry as set forth in claim 1 where said comparatormeans compares a predetermined fraction of the average voltage over apredetermined time period from the end-of-ribbon sensor with the realtime voltage from the end-of-ribbon sensor.
 4. Circuitry as set forth inclaim 1 further including an output limiting means for receiving theoutput signal from said comparator means.
 5. Circuitry as set forth inclaim 4 further including a sampling circuit for receiving the outputsignal from said output limiting means wherein said sampling circuit issampled for a predetermined period of time after a print hammermechanism in the printer has been activated.
 6. Circuitry as set forthin claim 5 further including a latch circuit for receiving the outputsignal of said sampling circuit, wherein said sampling circuit providesan output signal to be acted upon by said printer.